Magnetic brakes are advantageous for braking rotation and controlling the torque of rotating shafts or other rotating components. For example, during the manufacture or processing of wire, foil, paper, film, or other material wound on a spool or roller, the material may have to be brought to a stop at a predetermined point, such as at end of the roll. In other applications, magnetic brakes may be used to maintain a constant tension on the material during winding and unwinding.
Friction brakes are often not well suited to these uses for several reasons. Friction brakes may not brake unevenly. Friction brakes also generate dust, wear out and require maintenance. Magnetic brakes are contact-less and largely avoid these problems, so that magnetic brakes are generally preferred in winding and unwinding systems. Hysteresis brakes are a common type of magnetic brake that have been in use for many years. However, hysterisis brakes have several drawbacks, including relatively low torque, hysteresis, non-linearity, low power dissipation and high cogging.
Relatively low torque is an inherent characteristic of hysteresis brakes. Hysterisis is a result where as the input current is increased by a certain amount, the output torque of the brake will increase, but when the input current is decreased by the same amount, the torque will not decrease by the same amount. Non-linearity refers to the torque output of hysteresis brakes being dependent on rotation speed. Low power dissipation results from the materials and design of conventional hysterisis brakes. Cogging refers to non-smooth rotation at low speeds caused by residual magnetism.
FIG. 15 shows an example of a prior art hysteresis brake having a drag cup 23′ of material that can be magnetized. Typically the drag cup 23′ is manufactured from a single sheet of steel that either is spun or deep drawn. The cup 23′ is mounted on a rotating shaft 17′ with a drag ring 12′ on the drag cup 23′ rotating in a gap between the poles of the magnet. Hysteresis brakes do not wear because the parts are not in contact, although the rotating shaft is supported on ball bearings. The hysteresis effect will produce a certain residual cogging when the drag ring 12′ is stopped with current applied to the coil 13′. This cogging is due to the residual magnetic fields in the drag ring 12′ which remain after the current in the coil 13 switched off. Accordingly, improvements in magnetic brakes are needed.